In general, natural gas (NG) is liquefied into a form of liquefied natural gas (LNG) for the convenience of storage and transport, etc. A conventional natural gas liquefaction system using a mixed refrigerant that is mixed with one or more refrigerant such as hydrocarbon, HFC, etc, is depicted in FIG. 1 and comprises a compressor 12, a plurality of heat exchangers 41 to 44, at least one throttling valve 31 or 32, and at least one vapor-liquid separator 21 or 22. First, the mixed refrigerant 11 is compressed by the compressor 12, and the compressed refrigerant stream is supplied to a first vapor-liquid separator (below to be also called as ‘a first phase separator’) 21. The compressed refrigerant stream is separated into vapor phase and liquid phase refrigerant streams (i.e. vapor and liquid streams) in the first phase separator 21. The liquid-phase refrigerant (below to be also called as ‘a liquid stream’) is expanded to cold refrigerant through a first throttling valve 31 to become an expanded refrigerant stream, and the expanded refrigerant stream passes through the first heat exchanger 41 to cool a natural gas 13 and the vapor-phase refrigerant (below to be also called as ‘a vapor stream’) supplied from the first phase separator 21 by indirect heat exchange therewith, then it returns to the low-pressure portion of the compressor 12. Meanwhile, the vapor stream supplied from the first phase separator 21 is pre-cooled by the expanded refrigerant stream supplied from the first throttling valve 31 as it passes through the first heat exchanger 41 as described above, before it is supplied to the second vapor-liquid separator (below to be also called as ‘a second phase separator’) 22 to be again separated into vapor phase and liquid phase refrigerant streams.
The liquid stream supplied from the second phase separator 22 is expanded to cold refrigerant through a second throttling valve 32 in the same manner as the first throttling valve 31 to provide an expanded refrigerant stream. Then, the expanded refrigerant stream cools natural gas by indirect heat exchange as it passes through the third heat exchanger 43 and returns to the compressor 12 via the second and first heat exchanger 42 and 41 in sequence. Meanwhile, the vapor stream supplied from the second phase separator 22 is further pre-cooled as it passes through the third heat exchanger 43 as described above, before it is supplied to the Joule-Thomson throttling valve 33 for final expansion. The expanded refrigerant stream further cools the cooled natural gas that is flowed in through the third heat exchanger 43 as it passes through the fourth heat exchanger 44. After that, for regeneration of the remaining cold source, the refrigerant from the fourth heat exchanger 44 passes through the third, second and first heat exchangers 43, 42, and 41 in sequence to be indirectly heat exchanged to cool the above-mentioned vapor stream and natural gas, before it returns to the compressor 12. Briefly, natural gas 13 is cooled by indirect heat exchange with the expanded refrigerant stream as it passes through the fourth heat exchanger 44 to become liquefied natural gas 14.
However, the throttling process using the Joule-Thomson valve increases entropy due to embedded irreversibility and this becomes the main cause for decreasing the efficiency of the whole refrigeration cycle. Among several expansion processes, the throttling process of two-phase flow at the throttling valve 33 with lowest temperature occupies a large portion of efficiency loss.